A conventional television (TV) signal contains a video signal and a second signal containing binary data. In the United states, for example, TV signals commonly carry closed caption data along with the NTSC video signal. In other countries, teletext information is carried along with a
video signal. Moreover, any future NTSC digital transmitting standards, such as "intercast," will allow greater amounts of data to be carried with NTSC video signals.
As multimedia personal computers (PCs) become more powerful, the capability of viewing TV in a window on a computer will become more commonplace. A multimedia PC typically has a video capture circuit for receiving the TV signal and for displaying it on the screen. The video capture circuit will also extract the closed caption or teletext information and allow it to be displayed on the screen.
In a low cost TV decoder implementation, the incoming video signal is sampled at four times the color subcarrier (4*Fsc). This frequency makes the video signal easy to decode. In addition, a TV decoder implementation sampling the incoming signa at 4*Fsc is quite effective at extracting closed caption information from an NTSC signal.
However, PAL and intercast signals carry bits at a much higher frequency than in a closed caption signal. Moreover, a sampling rate of 4*Fsc is not a convenient multiple of the bit rate of, for example, teletext information in the PAL standard. Thus, sometimes the decoder will get two samples per bit and sometimes it will get three samples per bit. Accordingly, it is very difficult to extract data from a PAL or intercast signal using a 4*Fsc sampling rate.
In the PC environment, prior art decoders use software to resample the teletext data stream. The software essentially "guesses" where the center of a bit is and whether the bit is a one or a zero. Unfortunately, such software requires valuable processing time to decode the data.
Therefore, there is a need in the art for an effective system and method for decoding teletext and other data formats using the 4*Fsc sampling rate.